. >>> the head of the cia brought down by a dark secret. tonight we are learning how the fbi stumbled upon his extramarital affair. thanks for staying with us. i'm will thomas. the resignation of general david petraeus has left a lot of questions about possible security breaches and the future of the benghazi hearings. fox's nicole collins starts our coverage tonight. >> reporter: cia director general david petraeus stepping down admitting he had an extramarital affair. the retired four star general saying in a letter to cia employees, "such behavior is unacceptable both as a husband and as the leader of an organization such as ours." >> he has led america through some very, very difficult times. i think many of us are surprised that these kinds of things happen. >> reporter: the general's affair with his biographer paula broadwell was revealed in the course of an fbi investigation. according to her website, she of embedded with him in afghanistan from july, 2010 to july, 2011. general petraeus has been married for nearly 40 years to holly